usb: dwc2: debugfs: add support for complete register dump

Dump all registers to take a complete snapshot of dwc2 state.
Code is inspired by dwc3/debugfs.c

Acked-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Mian Yousaf Kaukab <yousaf.kaukab@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>